In the fields of spectroscopy and surgery, optical fibers employing laser inputs are increasingly being used. For surgery, optical fibers are often used in illumination of body cavities, imaging those cavities and in delivering laser energy for incision/excision, coagulation, homeostasis, and vaporization of tissue. Typically, the optical fibers which are used require a relatively high Numerical Aperture (NA) in order to capture as high a percentage as possible of the optical energy available from the laser source. High NA fibers, however, result in relatively wide divergence of the light spots at a relatively short distance from the ends of the fibers. Such divergence is not permissible for many applications; so that relatively expensive and cumbersome lens systems have been attached to the output ends of such fibers in order to focus or collimate (or nearly collimate) the light exiting from the output end of the fiber. Such lenses must be added to the fiber end as a separate manufacturing step, and tend to cause the endoscope (or spectroscopic probe tip) to be larger and more invasive than would be the case if such lens systems were not required.
Surgical fibers for energy delivery often are damaged in use, due to inadvertent contact with the target tissues. Contamination of the fiber output with tissue causes localized heating and consequent damage to the fiber, reducing the output beam quality. The wide-angle divergence of energy from high NA fibers contributes to this failure, in that the surgeon, in his search for the energy density he desires for the sought tissue effects, often inadvertently overshoots. This results from the fact that the high energy densities are found only very close to the fiber output; so unintended fiber/tissue contact is likely.
With lower NA output of a fiber, energy densities do not fall off as quickly; so that fiber/tissue separation of greater distances can be attained. Lower NA fibers are often incompatible with the launch NA of laser sources (and other, e.g. white light sources) used. A common additional problem is the minimum focal spot size of sources being larger than the optimum fiber core diameter. Typically, tapered fibers are used where the desired fiber is smaller than the minimal launch focal spot. While inefficient (typically 65%), these arrangements are often acceptable in many applications.
A popular pulsed Holmium doped yttrium-aluminum Garnet crystal laser (Ho:YAG), used in laser lithotripsy has a minimum focal spot size of approximately 300 .mu.M diameter. 300 .mu.M core fiber, however, is often too stiff to reach easily through small, highly twisted lumen of the type encountered in a human ureter, the location of the calcium carbonate kidney stones that lithotripsy is designed to treat. The maximum power of the laser, however, exceeds the minimum energy required to break up the stones; so that a surgeon is content with inefficient delivery if some means can be devised to get at least some significant portion of the laser energy into a smaller core fiber. It is desirable to use a smaller core fiber in order to achieve the flexibility not attainable with a 300 .mu.M core diameter.
Other applications, such as assemblies for performing diagnostics, for example, identification of cancerous versus non-cancerous tissues by Raman spectroscopy also are increasingly utilized. In spectroscopy, several basic configurations exist with applications in absorption/transmission, and fluorescence (including phosphorescence and Raman spectroscopy). A single fiber may be used to deliver and collect reflected or scattered energy when external optics are used to split the signal return and the excitation signal.
The basic fiber configuration typically includes a relatively high NA excitation fiber, which is uniform throughout its length. The path length for the absorption spectroscopy measurement then is determined by mounting the fiber in a threaded carrier tube, with a mirror on an attaching cap spaced a distance one-half that of the desired path (due to the reflection of the mirror causing the light to transverse the space twice) . Ideally, collimated or nearly collimated light (consistent with low NA fiber) is desired from the exit end of the excitation fiber; so that a maximum return of light is available for the return path. However, this is inconsistent with high NA fibers designed to capture the maximum light energy available from the source.
In spectroscopy, dual fiber devices or multiple fiber devices also may be employed, with one fiber being used as the excitation or illumination fiber and the others arranged in close proximity or surrounding the excitation fiber comprising the detection or collection fibers. Many of the same problems which exist with surgical applications also apply to these fiber optic spectroscopy devices. At the output end of the excitation fiber, it is desirable to have the light exit in a collimated or near collimated form. For high NA fibers, however, a relatively wide angle of light rays exit the end of the fiber; so that there is a relatively large circle of light or scattering at a relatively short distance from the fiber end. To overcome this, separate-lens systems may be applied to the end of the fiber. These lens systems present additional complications in spectral performance in addition to those previously noted.
It is desirable to provide an optical fiber capable of NA compression or reduction of the excitation fiber output, which is simple to manufacture, and which effectively reduces the NA between the input end of the fiber and the output end.